1. Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet and a method for producing it. In particular, the invention is to provide a thermal transfer image-receiving sheet excellent in transfer density, free from pressure influence on the transfer density and excellent in image storability after printing, and to provide a method for producing it.
2. Description of the Related Art
Heretofore, various thermal transfer recording methods are known. Above all, a dye diffusion transfer recording system is specifically noted as a process capable of producing color hard copies of which the image quality is the nearest to that of silver salt photographs. Moreover, as compared with silver salt photographs, the system has other various advantages in that it is a dry system, it can produce visible images directly from digital data, and it is simple in image duplication.
In the dye diffusion transfer recording system, a colorant (or dye)-containing thermal transfer sheet (hereinafter referred to as “ink sheet”) and a thermal transfer image-receiving sheet (hereinafter referred to as “image-receiving sheet”) are put one upon another, and thermal transfer sheet is heated with a thermal head from which the heat generation is ontrolled by electric signals given thereto, whereby the colorant in the thermal transfer sheet is transferred onto the thermal transfer image-receiving sheet for image information recording thereon. In this, three colors of cyan, magenta and yellow, or four colors of these and black are recorded as superimposed, thereby giving a color image having a continuously changing color density in a mode of transfer recording. In the dye diffusion transfer process, the ink sheet and the image-receiving sheet must be kept in close contact with each other. Accordingly, for obtaining high-quality prints having high image uniformity, the image-receiving sheet is required to have good smoothness.
With the recent development of digital imaging technology by computer, the quality of recorded images is bettered and the market of the dye diffusion transfer recording system is expanding, and with that, the demand for high-speed and high-density printing systems is increasing.
The thermal transfer image-receiving sheet for the system comprises, as formed on a support, an image-receiving layer on which the transferred colorant is fixed. It is known that a heat-insulating layer containing hollow particles is formed on the support to thereby enhance the colorant transferability owing to the heat-insulating effect of the pores that the hollow particles have. JP-A 2008-246943 discloses a method for producing an image-receiving sheet having a high print density and good smoothness according to a simultaneous multilayer coating method. The simultaneous multilayer coating method is a production method of excellent producibility for image-receiving sheets, in which gelatin is effectively used. This is because gelatin has the property of readily gelling, and it enables high-speed coating by cooling immediately after coating with it, therefore preventing interlayer color mixing. However, the film after gelled shrinks owing to water vapor evaporation, and therefore the hollow particles in the heat-insulating layer are crushed. Consequently, there is a problem in that the hollow particles could not fully exhibit the heat-insulating property thereof.
JP-A 2008-296528 discloses an image-receiving sheet having a high image density and good image uniformity, in which the heat-insulating low-density layer contains hollow particles, a hydrophobic resin adhesive and a hydrophilic adhesive. As the hydrophobic resin adhesive, used are a polybutadiene resin and a styrene-butadiene-acrylonitrile copolymer resin having a glass transition temperature of not higher than 10° C.; and as the hydrophilic adhesive, used is polyvinyl alcohol. The publication shows the effect of the sheet that yields little by pressure.
However, when the composition is applied to simultaneous multilayer coating method, high-speed coating with it is difficult and the composition could not fully prevent trouble of interlayer color mixing. When gelatin is used as the hydrophilic adhesive for gellation, the hollow particles are much crushed and the image density greatly lowers.
Consequently, it is desired to provide a thermal transfer image-receiving sheet having good transferability based on the heat-insulating property of the hollow particles therein and having high smoothness based on the advantage of the simultaneous multilayer coating method.